This invention relates to two-part surgical cements. More particularly, the invention relates to surgical cements useful in dentistry.
Surgical cements used in dentistry are generally composed of a powder and a liquid which, when mixed together, react to form a workable plastic mass which subsequently hardens to a coherent cement. The cement is useful in producing a solid union between two surfaces (for example, to retain metallic and non-metallic castings in a prepared tooth, or to adhere crowns, bridges and orthodontic appliances to the tooth). Depending upon the chemical composition of the surgical cement it may also be used as a cavity liner or base, as a temporary tooth restoration, as a root canal filling, or for pulp capping. The primary requisites for a dental cement are adequate strength, adhesion to the tooth, resistance to erosion in oral fluid, and a low level of irritancy to tooth pulp.
Early surgical cements used in dentistry were composed of lime and concentrated phosphoric acid and were characterized by a number of disadvantages. For example, they were highly irritating and injurious to tooth pulp and they were also very soluble in oral fluid. Later zinc oxide, and then calcined zinc oxide, were substituted for the lime. This resulted in improved compressive strength but the cement still suffered from poor adhesion to the tooth and was also irritating to the pulp.
Another widely used dental cement was based on zinc oxide-eugenol combinations which were not harmful to the pulp. However, such compositions had very low compressive strength and were not resistant to abrasion.
A more recent dental cement is a polycarboxylate cement of the type described in U.S. Pat. Nos. 3,655,605; 3,751,391; 3,804,794 and 3,814,717; and British Pat. No. 1,139,430. This type of dental cement has exhibited good adhesion to the tooth surface and is quite insoluble when cured.
In attempts to improve the compressive strength of the polycarboxylate dental cements, copolymerized carboxylic acids were utilized, e.g., as described in U.S. Pat. Nos. 3,804,794; 3,882,080; 3,962,267 and 4,016,124. In spite of the use of a copolymer, the inherent disadvantages of a polycarboxylate cement, i.e., low compressive strength and mixing difficulty, still remained.
Typically the polycarboxylate component of previously described cements is in the form of a liquid (although it may be in the form of a powder) which must be mixed thoroughly with the powder component (e.g., metal oxide) at the time of use to form a creamy paste. This creamy paste is then placed into the crown or bridge appliance which is then applied to the prepared site in the mouth. The paste is permitted to harden and excess material is removed. However, it is difficult to thoroughly and quickly mix the powder compoent and the liquid component. Furthermore, the dental cement begins to harden within 1.5-2.5 minutes and is completely set within about 8 minutes. Although various techniques may be used to incorporate the powder into the mixture in increments, the required mixing time is still somewhat prolonged. As a result, the dentist's working time with the dental cement is seriously shortened. Some have suggested that the two components be mixed on a chilled glass slab in order to slow the reaction, but if the chilled slab is below the dew point there can be condensation of moisture thereon to introduce even further problems. Moreover, because of the necessary haste in mixing the two components they are seldom proportioned accurately for optimum results.
The present invention overcomes the common deficiencies inherent in the two-component (liquid and powder) dental cement compositions.